The invention concerns a low-noise preamplifier with an electronic amplifying element having an input and an output, and with a first transformer having a primary winding and a secondary winding.
A preamplifier of this type has been disclosed by the circuit diagram of a preamplifier in NMR spectrometer CXP 100 of the company Bruker in 1980.
Preamplifiers are used in all electronic fields to amplify signals, in particular currents, to a level suited for a downstream connected electronic component. One fundamentally tries to keep the signal-to-noise ratio large during the amplifying process. The quality factor thereby used is the so-called noise factor.
For the amplification of alternating current signals, the input impedance of the preamplifier must also be taken into consideration. Standardized input impedances of 50 ohms or 75 ohms are usually desired to eliminate reflections, signal losses and resonance detunings e.g. from signal sources.
The conventional preamplifier arrangements normally utilize transistors in the emitter circuit configuration. When GASFETs (gallium arsenide based field effect transistors) are used, very good noise factors in the range of 0.3 dB can be obtained for a narrow frequency range. However, the input resistance of the GASFET preamplifier arrangement is complex, having a real portion of only 1 to 3 ohms. In other preamplifiers which can be used in a broadband range, defined input impedances of e.g. 50 ohms can be established, however, the noise factors which can be obtained are quite high, i.e. approximately 1.2 dB. To adjust the input impedance, a so-called λ/4 transformation (preamplifier decoupling) can be effected. Also known in the art are low-noise preamplifier arrangements having high input impedances in the kohm range.
The circuit diagram of the CXP 100 discloses a preamplifier having the input signal of the preamplifier connected to the emitter of a transistor via the secondary winding of a transformer. The transistor base is capacitively connected to ground, and the collector of the transistor is connected to a direct voltage source via the primary winding of the transformer. The output signal of the preamplifier is tapped from the primary winding of the transformer.
In contrast thereto, it is the object of the present invention to provide a preamplifier with extremely low noise which can be operated at basically any input impedance, in particular, at 50 or 75 ohms.